1. Field of the Invention
This invention generally relates to light beams and in particular to multi-wavelength lasers and continuous spectra light sources.
2. Related Art
A number of optical techniques may be used to obtain information about materials. One such technique is Raman spectroscopy. In Raman spectroscopy, laser light is incident on a surface of a material to be analyzed. Most of the light scatters elastically from the surface (which is referred to as Rayleigh scattering). However, some of the light interacts with the material at and near the surface and is scattered inelastically due to excitation of vibrational, rotational, and/or other low-frequency modes of the material. Detecting the frequencies of such vibrational states yields information about the molecular structure and quality of the material. The inelastically scattered light is shifted in wavelength with respect to the incident laser light, either down in frequency (corresponding to the excitation of a material mode by the incident photons, also referred to as Raman Stokes) or up in frequency (corresponding to the interaction of the incident photons with an already-excited material mode, also referred to as an anti-Stokes Raman). The amount of the shift is independent of the excitation wavelength, and the Stokes and anti-Stokes lines are displaced from the excitation signal by amounts of equal magnitude.
Many laser and other optical sources that are used to analyze materials using spectroscopy are known to generate a plurality of spectral lines. This can be used advantageously. For example, many semiconductor electronic devices consist of numerous layers of semiconductor materials with varying compositions and constituents. Often, these layers are Raman active, and the magnitude of the wavelength shift of the Stokes and anti-Stokes lines relative to the excitation wavelength are dependent on the stoichiometry of the chemical composition and crystalline properties of each layer.
As is well known in optics, because the index of refraction of semiconductor materials is dispersive, different probe wavelengths have different penetration depths into the bulk of the material. It is often the case that such penetration depths are comparable to the depths to which various layers of materials are prepared. Thus, use of laser light of different excitation wavelengths will probe the Raman scattering properties of materials at different depths. It is therefore advantageous to have a system capable of simultaneously providing laser excitation lines to obtain, via Raman spectroscopy, information about the quality and stoichiometry of layers at various depths of material, within the limits of penetration of the laser lines.
Other applications can also be envisioned. For example, in liquid samples containing biological specimens, choice of excitation wavelength affects the generation of fluorescence, which is another means for investigation of biological materials.
One problem that arises in this connection is the means for selecting the preferred set of wavelengths, deleting or excluding others, and combining them into a single beam for optical characterization of material properties. For example, an argon ion laser has more than thirteen laser lines, all of widely varying relative output.
FIG. 1 shows a simplified view of a conventional means for selecting and combining different wavelengths originating from several different sources, such as lasers, multi-wavelength lasers, or continuous spectra sources. These sources can be identical, and therefore produce the same laser lines, or they may be different, and thus provide a spectrum of wavelengths. In this approach, a bank of light sources provides multiple light beams 105, each at multiple wavelengths of laser output or continuous spectra, to a bank of bandpass filters 115, referred to collectively as a stack. Beams of single wavelengths pass through each filter 115, which limits the wavelength or wavelengths that transmit. The several beams then pass to a stack of beam splitters 110 or partial mirrors that behave in a manner reciprocal to beam splitting, i.e., they combine the beams into a single beam 125 that exits the system at the bottom of the figure. Mirrors 120 are strategically placed to recoup some fraction of the laser beams that would otherwise be lost. However, this is a highly inefficient system from the point of view of wasted beam energy, power consumption and duplication of costly laser hardware. Furthermore, optical alignment to combine beams from a plurality of separate lasers or other light sources requires costly precision alignment.
Therefore, there is a need for forming light beams by means of using least a multi-wavelength source in which the desired output wavelengths can be chosen with minimum loss of energy and ease of selection.